Method of fabricating a composite structure with a stable bonding layer of oxide

ABSTRACT

A method of fabricating a composite structure that has at least one thin film bonded to a support substrate and a bonding layer of oxide formed by deposition between the support substrate and the thin film. The thin film and the support substrate have a mean thermal expansion coefficient of 7×10 −6  K −1  or more. The bonding layer of oxide is formed by low pressure chemical vapor deposition (LPCVD) of a layer of oxide on the bonding face of the support substrate or on the bonding face of the thin film. The thin film has a thickness of 5 micrometers or less while the thickness of the layer of oxide is equal to or greater than the thickness of the thin film.

TECHNICAL FIELD AND PRIOR ART

The present invention relates to the fabrication of composite structurescomprising at least one support substrate onto which a thin film isbonded via a deposited bonding layer of oxide. That type of structure isparticularly intended for use in the field of microelectronics,optoelectronics, and optics to produce, by epitaxial growth,semiconductor materials such as those of type III/V, in particularbinary, ternary, or quaternary type III/N materials, such as GaN, AlGaN,InGaN, or InAlGaN.

That type of composite structure is generally produced using thewell-known Smart Cut® technique that consists of:

-   -   subjecting a source substrate or donor substrate to ion        implantation to create a zone of weakness at a certain depth in        the substrate;    -   bonding (by wafer bonding) the face of the donor substrate that        has undergone implantation with a support substrate or        “receiving substrate”; and    -   detaching the donor substrate by fracture at the zone of        weakness to transfer the portion located between the face that        has undergone implantation and the zone of weakness of the donor        substrate to the receiving substrate, the transferred portion        constituting the thin film of the composite structure.

In order to facilitate detachment of the thin film from the sourcesubstrate, good bonding between the source substrate and the supportsubstrate is necessary. It is ensured by a bonding interface producedbetween two bonding layers that then endows the composite structure withgreat stability even with materials with differing coefficients ofthermal expansion. The bonding layers correspond to layers of oxideformed on the faces of the source substrate and the support substrateintended to be brought into intimate contact. These oxide layers act asplanarization layers that encourage intimate contact of the substratesduring wafer bonding. The composite structure may then undergo processesinvolving large variations in temperatures, such as epitaxial growth,without being deteriorated.

The fabrication of a composite structure for epitaxial growth may alsobe carried out using a step of bonding the source substrate and asupport substrate followed by a chemical thinning step or a mechanicalpolishing step carried out on the source substrate, to reach the desiredthickness of the thin film. That type of fabrication also requiresbonding that is highly temperature-stable and, as a result, the use of abonding layer of oxide to guarantee a good bonding interface.

With a material formed from silicon or silicon carbide (SiC), thesubstrate is oxidized by heating in an appropriate atmosphere to obtaina bonding layer of oxide of silicon (SiO₂) that is termed “thermaloxide”, that corresponds to a stoichiometric oxide, that is dense (SiO₂is defined as dense when it is slowly attacked by a solution of HF), andthat remains stable at high temperature.

When bonding two substrates, one of which is a material that cannotproduce such a layer of oxide of silicon by thermal oxidation, as with asapphire substrate, for example, it is necessary to form the oxide ofsilicon by deposition using a technique such as plasma enhanced chemicalvapor deposition (PECVD) or low pressure chemical vapor deposition(LPCVD). This is then a “deposited oxide”, that has a composition thatis different from stoichiometric SiO₂ obtained by thermal oxidation (theoxide deposited has a composition of the type Si_(x)O_(y)H_(z)). Thisdeposited oxide is less dense and does not have the same properties asthermal oxide. A heat treatment may be carried out in order to densifyit and to approach the properties of the thermal oxide. However, evenafter densification annealing, the deposited oxide may still betemperature-unstable, in particular during treatments carried out attemperatures that are higher than the oxide deposit temperature.

When a composite substrate, formed by a sapphire support substrate onwhich a thin layer of sapphire is laid, is heated to prepare it for GaNepitaxy, for example, microcavities appear once the oxide depositiontemperature is exceeded. The microcavities appear exclusively at thebonding layer of deposited oxide of silicon. They are visible at thesurface of the thin film of sapphire by the formation of irreversibleblisters that “buckle” its surface and render it unsuitable for epitaxybecause the surface is no longer smooth and the lattice parameter isdistorted. The microcavities in the deposited oxide layer appear anddevelop more rapidly as the temperature is raised.

As a function of their density and their size, such microcavities maygenerate a zone of weakness in the bonding layer and the operationssubsequently carried out on the structure will result in delamination ofthe thin film. Such microcavities may even affect the entire volume ofthe oxide layer and cause it to rupture and, consequently, causedetachment of the thin film from the support substrate. Thus, whetherduring fabrication of the composite structure or during subsequentepitaxy, the presence of such microcavities always results indelamination of the thin film.

In order to illustrate this problem, the Applicant carried out thefollowing experiment. A source substrate of sapphire, comprising a zoneof weakness such that the thin film to be transferred had a thickness of0.6 μm [micrometer], was bonded to a sapphire support with a bondinglayer of oxide deposited by PECVD at 300° C. to a thickness of 0.3 μm.To this end, the two substrates were brought into intimate contact atambient temperature, then a splitting heat treatment was appliedfollowed by bonding reinforcement annealing at 1100° C. for 3 hours.Total delamination of the transferred sapphire film was then observeddue to detachment within the deposited oxide layer.

Document EP-A-0 898 307 describes a method of unbonding a wafer from asupport at the bonding interface by using an oxide bonding layer formedby PECVD. That oxide has the peculiarity of having OH species thatdiffuse to the bonding interface under the action of a heat treatment(600-1350° C.) carried out after bonding stabilization annealing and thedesired treatments on the integrated circuits. Said species developuntil a gas is formed and they then form bubbles that diffuse and areconcentrated locally at the bonding interface, i.e. the interfacelocated between the oxide bonding layer deposited on the wafer and thesupport substrate. That phenomenon encourages weakening of the bondinginterface until the support substrate unbonds completely from thebonding layer, the bonding layer initially deposited on the waferremaining integral with the wafer.

However, in contrast to what is described in patent document EP-A-0 898307, which describes a concentration of gaseous species and bubbles atthe bonding interface, the microcavities that are to be prevented in thepresent invention are formed throughout the deposited oxide layer andwhen the temperature of the heat treatment applied exceeds that forcreep of the oxide subjected to a high stress (>100 MPa [megapascals]),i.e. in the range approximately 800° C. to 1200° C. for layers of SiO₂not intentionally doped and deposited by LPCVD or PECVD techniques. Saidmicrocavities have the effect of deforming the surface of thetransferred thin film, which impedes certain applications such asepitaxy. Further, the delamination of the film observed by the Applicantderives from detachment within the oxide layer, the bonding layer notbeing unbonded at the bonding interface but ruptured by themicrocavities developed throughout its volume.

Document US-2006/0255341 describes the fabrication of a compositesubstrate by transfer of a seed thin film onto a support substrate,intended for epitaxial growth of III/N materials. Direct bonding(without an oxide layer) of a thin layer of sapphire onto a supportsubstrate that has a lower thermal expansion coefficient requires a highpressure that causes a folding phenomenon in the thin layer. Transfer ofthe layer of sapphire requires implantation of a large dose of ionicspecies, thereby creating a stress gradient through the thickness of thetransferred layer. When the stress increases due to the bondingconditions, the material deforms, and allows the stress to be relieved.To remedy that, the document describes the use of a layer of sapphirewith a thickness of the order of 800 nm [nanometer] to enhance themechanical strength of the layer.

The document also describes, during a transfer process, a phenomenon ofdelamination of the thin layer of sapphire, which is a rigid material,due to stress induced by implantation, by the difference in the thermalexpansion coefficients of the materials, and by its surface that isfairly inert chemically speaking and does not tend to form covalentbonds with other material surfaces. To overcome that, it is possible touse plasma treatment to activate the surface, a bonding layer of SiO₂,Si₃N₄, AlN, and/or an adhesive layer.

SUMMARY OF THE INVENTION

The invention aims to overcome the above-mentioned disadvantages andproposes composite structures wherein the thin film and supportsubstrate have a mean coefficient of thermal expansion that is 7×10⁻⁶K⁻¹ or more in the range of temperatures to which the structure will besubjected, for example from 20° C. to 1200° C., and wherein the bondinglayer comprises at least one deposited layer of oxide of silicon thatremains stable even at high temperature. The invention aims to allow thefabrication of such composite structures in order to avoid the formationand development of microcavities in the deposited oxide layer resultingin deformation of the thin film and in its delamination during heattreatments applied during the fabrication of the composite structure andto allow the formation of materials by epitaxy starting from thecomposite structure.

This aim is accomplished by means of a method of fabricating a compositestructure in which the bonding layer of oxide of silicon is formed bylow pressure chemical vapor deposition (LPCVD) of a layer of oxide onthe bonding face of the support substrate and/or on the bonding face ofthe thin film, while the thickness of the thin film is 5 micrometers orless and the thickness of the layer of oxide of silicon is equal to orgreater than that of the thin film.

As explained below in detail, using a bonding layer formed by LPCVDdeposition, wherein the thickness is greater than that of the thin film,prevents plastic (irreversible) deformations within the bonding layer.Thus, the formation of microcavities in the deposited bonding layer ofoxide of silicon is avoided, even during heat treatments at hightemperature (in particular higher than 900° C.).

Following a study, detailed below, on the temperature behavior of layersof oxide of silicon as a function of the deposition technique employed,the Applicant has determined that deposition by LPCVD can produce oxideswith temperature stability that is close to that of oxides of siliconobtained by thermal oxidation. Further, to resist stresses due to thepresence of materials with expansion coefficients that are high comparedwith that of the oxide layer in the composite structure, the thicknessof the oxide layer is greater than or equal to that of the thin filmthat is limited to 5 micrometers. This means that the stresses appliedat the bonding layer during high temperature treatments can be reduced,as well as the risk of plastic deformations occurring in the bondinglayer.

The material of the bonding layer of oxide of silicon formed by lowpressure chemical vapor deposition may be produced using different knownprecursors such as silane, dichlorosilane, or TEOS (tetra-ethylorthosilicate).

In accordance with one aspect of the invention, prior to bonding, themethod further comprises a step of densification heat treatment of thelayer of oxide of silicon deposited by low pressure chemical vapordeposition on the bonding face of the support substrate and/or on thebonding face of the thin film. This densification heat treatment canfurther increase the temperature behavior of the deposited oxide asregards the formation of microcavities. This step can, if necessary,reduce the ratio of the thickness of the deposited oxide layer to thethickness of the thin film.

The heat treatment step is carried out at a temperature that is higherthan the temperature at which the oxide bonding layer is deposited.Optionally, the heat treatment may be carried out in a neutral oroxidizing atmosphere, for example.

In accordance with a particular characteristic of the method of theinvention, the thin film may be obtained using the Smart Cut® technique.The method then further comprises:

-   -   a step of implantation by bombardment of one face of a donor        substrate using ions to form, at a predetermined depth in the        substrate, a layer of weakness defining the thin film in the        upper portion of the substrate;    -   a step of bonding by placing the donor substrate in intimate        contact with the support substrate;    -   a step of detachment of the thin film in contact with the        support substrate by splitting at the layer of weakness formed        in the donor substrate.

In accordance with a particular characteristic of the invention, thethin film may be produced with:

-   -   a step of bonding by bringing a donor substrate into intimate        contact with the support substrate;    -   a step of chemical or mechanical thinning of the donor substrate        to form the thin film.

After the bonding step, a step of bonding stabilization annealing may becarried out at a temperature of more than approximately 900° C. withoutmicrocavities appearing in the deposited layer of oxide.

The invention also provides a method of producing at least one layer ofsemiconductor material, in particular materials of the binary, ternary,or quaternary III/V and III/N types, such as GaN, AlGaN, InGaN, orInAlGaN, by epitaxial growth on a composite structure fabricated inaccordance with the fabrication method described above, epitaxial growthbeing carried out from the thin film of the composite structure thatforms a crystalline seed layer for growth.

In accordance with one aspect of the invention, epitaxial growth iscarried out over a predetermined period corresponding to the formationof a layer of semiconductor material with a thickness that is sufficientto be self-supporting alone, namely a thickness of at least 100 μm, thatallows removal of the composite structure after epitaxy. In a variation,the crystalline seed layer for growth may be preserved with the layer ofepitaxially grown semiconductor material to form a self-supportingstructure that may be used for repeated epitaxy. Epitaxial growth of thelayer of semiconductor material is then carried out for a predeterminedperiod that allows a cumulative thickness of the seed layer andsemiconductor layer of at least 100 μm to be produced.

In accordance with a further aspect of the invention, epitaxial growthis carried out for a predetermined period corresponding to the formationof a layer of semiconductor material with a thickness of at least 10 μm,before any annealing of the composite structure, which corresponds to athickness sufficient to withstand the conditions for a new epitaxialgrowth step in the same epitaxy equipment provided that the layerobtained does not have to be manipulated. When the crystalline seedlayer is preserved (only the support is detached), growth is carried outfor a period that can produce a cumulative thickness of the seed layerand the semiconductor layer of at least 10 μm.

The invention also envisages a composite structure produced inaccordance with a method of fabricating a composite structure asdescribed above.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A to 1I are diagrammatic sectional views showing the productionof a composite structure and epitaxy in accordance with oneimplementation of the invention;

FIG. 2 is a flowchart of the steps carried out in FIGS. 1A to 1I;

FIGS. 3A to 3D are diagrammatic sectional views showing the productionof a composite structure and epitaxy in accordance with anotherimplementation of the invention;

FIG. 4 is a flowchart of the steps carried out in FIGS. 3A to 3D.

DETAILED DESCRIPTION OF IMPLEMENTATIONS OF THE INVENTION

The present invention is generally applicable to the production ofcomposite structures comprising at least one thin film bonded to asupport substrate via a bonding layer formed by deposition between thesupport substrate and the thin film, the thin film and the supportsubstrate having a thermal expansion coefficient of 7×10⁻⁶ K⁻¹ or moreover a temperature range extending from ambient temperature (20° C.) to1200° C.

The Applicant has observed that the phenomenon of microcavity formationin the bonding layer of oxide of silicon in the presence of hightemperatures occurs when the bonding layer is a layer of oxide formed bydeposition and the composite structure comprises one or more materialswith a thermal expansion coefficient of 7×10⁻⁶ K⁻¹ or more. Themicrocavities are larger when the thermal expansion coefficients of thematerials used are high at the temperature reached during fabrication ofthe composite structure or during its subsequent use (epitaxy). Theformation and development of microcavities within the deposited oxidelayer has been observed at temperatures above the oxide depositiontemperature. The appearance of microcavities in the oxide may beexplained by the transition from an elastic mode of deformation to aplastic mode of deformation, which is thus not reversible. Elasticdeformation is characterized by a modification of the initial state ofthe oxide, for example under the action of a mechanical stress, and itsreturn to the initial state once the stress is withdrawn. Plasticdeformation results in irreversible deformation after which the oxidecannot regain its initial state once the mechanical stress is withdrawn.The transition to plastic deformation occurs when a stress threshold isreached in the oxide. This transition is linked to temperature, to thecreep characteristics of the oxide and to the stress applied by thedifference in expansion of the bonded materials and the oxide. The creeptemperature of the oxide is one of the characteristics of the oxide thatindicates the temperature beyond which the oxide changes from an elasticdeformation mode to a plastic deformation mode when it is not subjectedto a stress. Temperature increases the rate of creep of the oxide. Thus,the level of stress to be applied to provoke the formation ofmicrocavities in the deposited oxide layer is lower when the oxide hasan intrinsic capacity to creep easily, especially due to the temperatureapplied vis-à-vis its creep temperature.

As a result, to prevent the formation and development of microcavitieswithin the deposited oxide layer in a composite structure as describedabove, it is necessary to use both a deposited oxide where creep isdifficult to provoke and to reduce the stresses applied to the oxideduring high temperature treatments.

The Applicant has studied the temperature behavior of oxides of siliconobtained by different deposition techniques and has discovered that abonding oxide deposited by low pressure chemical vapor deposition, alsotermed LPCVD, can reduce its capacity to creep. The experiments carriedout by the Applicant have shown that the properties of the oxide have asubstantial influence on the formation of microcavities in the bondinglayer and that these properties can be influenced by the depositiontechnique employed.

The following three types of oxides of silicon, formed using differenttechniques and different gaseous precursors, were tested for bonding ofa thin film of sapphire onto a sapphire support substrate:

-   -   oxide of silicon produced from a silane precursor, deposited at        300° C. by plasma enhanced chemical vapor deposition or PECVD;    -   oxide of silicon produced from a silane precursor deposited at        800° C. by LPCVD (also termed HTO silane, HTO meaning high        temperature oxide) (if the oxide is deposited by PECVD, and thus        at a lower temperature, it is not termed HTO silane);    -   oxide of silicon produced from a dichlorosilane (DCS) precursor        deposited at 900° C. by LPCVD (also termed HTO DCS).

Microcavity formation is less significant with HTO silane and HTO DCS.In contrast, microcavity formation is more significant with oxide ofsilicon deposited by PECVD deposition. It thus appears that creep of thedeposited oxides is more difficult to provoke when they are deposited bythe LPCVD technique.

Furthermore, since the formation of microcavities is linked to thestress due to high thermal expansion coefficients of the materials ofthe structure, the invention also proposes reducing that stress byforming (by transfer or by mechanical or chemical thinning) a thin filmthickness of 5 μm or less and of forming a deposited layer of oxide forbonding with a thickness that is greater than or equal to that of thethin film. Thus, mechanical stresses, derived from the difference inexpansion of materials during thermal treatments carried out on thestructure, are limited so that they do not exceed the plasticdeformation (creep) threshold of the oxide at the temperature underconsideration.

As an example, the Applicant has carried out tests that showed that whena film of sapphire with a thickness of 0.5 μm is transferred onto a 0.3μm thick bonding layer of HTO silane oxide deposited by LPCVD,delamination of the film took place after a heat treatment carried outat 1100° C. for 3 hours. In contrast, when a 0.3 μm thick film ofsapphire was transferred onto a bonding layer that was also 0.3 μm thickof HTO silane oxide deposited by LPCVD, neither delamination normicrocavities appeared after a heat treatment carried out at 1100° C.for 3 hours.

In general and in accordance with the invention, the higher the thermalexpansion coefficient of the material of the thin film, the thinner itmust be relative to the thickness of the deposited oxide layer. As anexample, the thickness of the thin film should be significantly reducerelative to that of the deposited oxide layer when the thin film isformed from lithium tantalate (LiTaO₃) that has a thermal expansioncoefficient of 16×10⁻⁶ K⁻¹ at ambient temperature. The skilled person isable to determine without particular difficulty the necessary reductionin the thickness of the thin film compared with that of the depositedoxide as a function of the thermal expansion coefficient of the materialof the thin film.

In addition, the temperature behavior of the deposited oxide may beimproved by using an oxide obtained by LPCVD deposition that has adensity that is as close as possible to that of the thermal oxide. Tothis end, a densification anneal may be applied to the oxide depositedby LPCVD prior to bonding.

With a composite structure comprising a 0.5 μm thick thin film ofsapphire on a sapphire support substrate with a 0.2 μm thick bondinglayer of HTO DCS oxide, a bonding stabilization anneal carried out at900° C. for 1 hour provokes complete delamination of the sapphire film.In contrast, when the HTO DCS oxide of the same composite structure isannealed for 30 minutes at 1200° C. in a nitrogen atmosphere (N₂) beforecarrying out bonding, a bonding stabilization anneal carried out at1050° C. for 1 hour does not lead to delamination of the thin film.

However, microcavities still appear and the surface of the transferredfilm is damaged. The quality and resistance to delamination of the thinfilm of the composite structure are thus not sufficient for subsequentapplications such as epitaxy. The oxide densification anneal reduces itspropensity to form microcavities but is not sufficient for the envisagedapplications. With materials with high thermal expansion coefficientssuch as sapphire, the stresses linked to differences in the thermalexpansion coefficients must also be reduced by increasing the ratio ofthe thickness of the oxide layer to the thickness of the transferredthin film as explained above.

Thus, a composite structure produced in accordance with the inventioncan withstand temperatures of more than 900° C. for the epitaxy ofmaterials such as materials of type III/N including GaN and otherternary or quaternary alloys, AlN, AlGaN, InGaN, AlGaInN, BGaN.Furthermore, the epitaxially grown layer may be composed of a stack ofthese various materials, in particular to constitute the active layersfor an LED or laser diode.

The composite structure of the present invention is particularlysuitable for materials with high expansion coefficients (TEC), i.e. amean of 7×10⁻⁶ K⁻¹ or more over a temperature range extending fromambient temperature (20° C.) to 1200° C. In particular, the structuremay comprise a thin film and/or the support substrate formed fromsapphire (Al₂O₃) (TEC of 7.5×10⁻⁶ K⁻¹), lithium tantalate (LiTaO₃) (TECof 16×10⁻⁶ K⁻¹), LiNbO₃ (TEC of 15×10⁻⁶ K⁻¹) and Haynes® 230® Alloy (TECof 11.8×10⁻⁶ K⁻¹), which is a commercial alloy primarily composed of Ni,Cr, Mo, W (Haynes® 230® Alloy is not used for the thin film when thefilm is intended for use as a seed layer for epitaxy), or MgO.

As is well known, as a function of the nature of the crystalline seedlayer for growth, various binary, ternary or quaternary III/V or III/Nsemiconductor materials may be formed. In particular, the compositestructure for epitaxy of the invention is intended for the epitaxialgrowth of GaN, InGaN, AlGaN, AlGaInN, and indium nitride (InN).

A method of fabricating a composite structure followed by a method ofproducing a layer of semiconductor material, here a III/N material, byepitaxy in accordance with an implementation of the invention isdescribed with reference to FIGS. 1A to 1I and 2.

Production of the composite structure for epitaxy commences bydepositing a bonding layer 12 on one face of a support substrate 10(step S1, FIG. 1A). In the example described here, the support substrate10 is formed from sapphire (Al₂O₃). The bonding layer 12 is a layer ofHTO silane deposited by LPCVD at a deposition temperature of 800° C. fora period that can deposit a thickness of HTO silane of approximately 0.5μm. The deposited oxide is then densified by applying a densificationanneal carried out at 1200° C. for 30 minutes in a nitrogen atmosphere(step S2).

The surface of the layers of bonding oxide 12 is planarized bychemical-mechanical polishing (CMP) to obtain a surface roughness ofless than 5 Å [Angstrom] RMS over a surface area of 5×5 μm² and therebyfacilitate subsequent intimate contact (step S3, FIG. 1B). Thus, afterpolishing, the layer 12 has a thickness of 0.3 μm±0.05 μm.

An oxide layer 13 may also be formed on a donor substrate 11 formed fromsapphire. The layer 13 is a layer of HTO DCS or HTO silane deposited byLPCVD at a deposition temperature of 900° C. for a period enabling athickness of HTO silane of approximately 0.2 μm to be deposited (stepS4, FIG. 1C). The layer 13 acts as a protective layer for implantation.

Next, implantation is carried out wherein the donor substrate 11undergoes ionic bombardment 20 with hydrogen ions H⁺ through the planarface 9 of the substrate comprising the oxide layer 13. Implantation ofH⁺ ions is carried out at an implantation dose in the range 1×10¹⁷atoms/cm² [atoms/square centimeter] to 4×10¹⁷ atoms/cm² and with animplantation energy in the range 30 keV [kilo-electron volt] to 200 keV.Implantation is carried out at a temperature in the range 20° C. to 400°C., preferably in the range 50° C. to 150° C. for a period of 1 minuteto 10 hours. These implantation conditions can create, at apredetermined depth in the donor substrate 11, a layer of defects or alayer of weakness 3 parallel to the face 9 of the substrate, defining athin film 4 in the upper region of the substrate 11 with a thickness of5 μm or less and at the thickness of the bonding layer, and also aportion 5 in the lower region of the substrate corresponding to theremainder of the substrate 1 (step S5, FIG. 1D). It is also possible touse ionic implantation of other species such as helium or argon, as wellas co-implantation combining two species such as hydrogen and helium.

Next, the protective oxide layer 13 is annealed (step S6, FIG. 1E). Inorder to eliminate the protective layer, a suitable chemical techniqueis employed that depends on the nature of the layer or layers to bewithdrawn. As an example, a protective layer of oxide of silicon isreadily removed by etching with a dilute 10% HF solution or usingmixtures known as BOE (buffered oxide etch).

Optionally, the surface of the layer 12 as well as the surface of thedonor substrate 11 may be exposed to plasmas based in particular onoxygen, nitrogen or argon that can activate the bonding surfaces andincrease their adhesive capacity (steps S7, S7′).

Next, bonding is carried out by bringing the face of the HTO silanelayer 12 into intimate contact with the face 9 of the donor substrate 11that has undergone implantation (step S8, FIG. 1F). Bonding is carriedout by wafer bonding. The principle of bonding by wafer bonding is knownper se and is not described in further detail. It should be recalledthat bonding by wafer bonding is based on bringing two surfaces intointimate contact, i.e. without the use of a specific material (adhesive,wax, low melting point metal, etc), the attractive forces between thetwo surfaces being high enough to provoke molecular bonding (bondinginduced by the ensemble of the attractive forces (Van der Waals forces)of electronic interaction between atoms or molecules of the two surfacesto be bonded).

The assembly of the two substrates then undergoes splitting annealing toprovoke fracture of the donor substrate 11 at the plane of weakness 3and transfer proper of the thin film 4 onto the support substrate 10(step S9, FIG. 1G). Splitting annealing is carried out at 650° C. for 5hours.

In addition, a bonding stabilization anneal is carried out at 1050° C.for 2 hours without deformations appearing on the surface of the thinfilm 4 (step S10).

The surface of the film 4 may then be prepared for epitaxy, for exampleby polishing the surface roughness, by light chemical etching (step S11,FIG. 1H) or plasma etching or any other surface treatment allowing thesurface to be prepared for epitaxy.

As can be seen in FIG. 1H, a composite structure 14 is obtainedcomprising the support substrate 10, an oxide bonding layer 12 of HTOsilane deposited by LPCVD and a thin film 4 of sapphire that may act asa crystalline seed layer for growth.

In the example described here, epitaxial growth of a layer of galliumnitride (GaN) 15 is carried out on the thin film 4 (step S12, FIG. 1I).Epitaxial growth is carried out at 1050° C. for 3 hours, for exampleusing hydride vapor phase epitaxy, HVPE. No microcavities nor anydelamination was observed after said epitaxy.

Another mode of fabrication of a composite structure in accordance withthe present invention is described with reference to FIGS. 3A to 3D and4.

In the implementation presented here, a bonding layer of oxide ofsilicon, for example HTO silane or HTO DCS, is deposited by LPCVD bothon the face of a support substrate 20 and on a face of a donor substrate21, both of sapphire (step S20, FIG. 3A). The support substrate 20 andthe donor substrate 21 each respectively comprise a bonding layer ofoxide 22 a, 22 b.

The layers of bonding oxide 22 a, 22 b are then polished to prepare forwafer bonding of the two substrates (step S21). Each layer 22 a, 22 bhas a final thickness of approximately 0.4 μm after polishing.

Once the layers 22 a and 22 b have been polished, they are brought intointimate contact to allow bonding of the two substrates by wafer bonding(step S22, FIG. 3B), the combination of layers 22 a and 22 b forming asingle deposited layer of oxide 22 with a thickness of approximately 0.8μm.

A bonding stabilization anneal is then carried out between 200° C. and800° C. for 1 to 5 hours (step S23).

The donor substrate 21 is thinned to a thickness of 0.3 μm (step S24,FIG. 3C). Said thinning is carried out by mechanical polishing of theexposed face of the donor substrate.

As can be seen in FIG. 3C, a composite structure for epitaxy 23 isobtained thereby comprising the support substrate 20, a bonding layer ofoxide 22 (HTO DCS or HTO silane) and a thin film 24 that results fromthinning of the donor substrate 21 and that can act as a crystallinegrowth seed for epitaxy.

The composite structure 23 may also undergo a second bondingstabilization anneal carried out at 1100° C. for 1 hour (step S25).

The surface of the thin film 24 may then be prepared for epitaxy bychemical mechanical polishing (CMP), light chemical etching and/orplasma etching with the aim of reducing its surface roughness (stepS26). Epitaxial growth of a type III/N layer of material 25 may then becarried out on the thin film 24 (step S27, FIG. 3D) without theappearance of microcavities nor of any delamination.

1.-16. (canceled)
 17. A method of fabricating a composite structurecomprising at least one thin film bonded to a support substrate, abonding layer of oxide formed between the support substrate and the thinfilm by deposition, with the thin film and the support substrate havinga mean thermal expansion coefficient of 7×10⁻⁶ K⁻¹ or more, wherein thebonding layer of oxide is formed by low pressure chemical vapordeposition (LPCVD) of a layer of oxide on either a bonding face of thesupport substrate or a bonding face of the thin film, or both, whereinthe thin film has a thickness of 5 micrometers or less and the bondinglayer of oxide has a thickness that is equal to or greater than thethickness of the thin film.
 18. The method of claim 17, wherein thelayer of oxide is an oxide of silicon formed using precursors of silane,dichlorosilane or tetra-ethyl orthosilicate.
 19. The method of claim 17,which further comprises, prior to bonding, conducting a densificationheat treatment of the layer of oxide.
 20. The method of claim 19,wherein the densification heat treatment step is carried out at atemperature that is higher than the temperature at which the bondinglayer of oxide is deposited.
 21. The method of claim 17, wherein thethin film is transferred to the support substrate by: implanting ions bybombardment of a face of the donor substrate to form, at a predetermineddepth in the donor substrate, a layer of weakness defining the thin filmbetween the donor substrate face and the layer of weakness; placing thedonor substrate face of in intimate contact with the support substrateto bond the two together; and detaching the thin film splitting at thelayer of weakness of the donor substrate.
 22. The method of claim 17,wherein the thin film is transferred to the support substrate by:bringing one face of a donor substrate into intimate contact with thesupport substrate to bond the two together; and thinning the donorsubstrate to leave only the thin film on the support substrate.
 23. Themethod of claim 22, which further comprises, after the bonding step,conducting a bonding stabilization annealing at a temperature of morethan approximately 900° C.
 24. The method of claim 17, wherein the thinfilm is approximately 0.3 micrometers thick.
 25. The method of claim 17,wherein the bonding oxide layer is approximately 0.4 micrometers thick.26. The method of claim 17, wherein the support substrate comprisessapphire, LiTaO3, LiNbO3, MgO, or an alloy of Ni, Cr, Mo and W.
 27. Themethod of claim 17, wherein the thin film comprises sapphire, LiTaO3,LiNbO3, or MgO.
 28. A method of producing at least one layer ofsemiconductor material, by epitaxial growth of the at least one layer ofsemiconductor material on a composite structure that is made accordingto the method of claim 17, wherein the epitaxial growth is carried outon the thin film of the composite structure.
 29. The method of claim 28,wherein the layer of epitaxially grown semiconductor material is a layerof a binary, ternary, or quaternary III/N material.
 30. The method ofclaim 28, wherein the epitaxial growth is carried out for apredetermined period corresponding to the formation of a thickness ofsemiconductor material or a cumulative thickness of the semiconductormaterial layer and the thin film of at least 10 micrometers.
 31. Amultilayer structure comprising a composite structure made according tothe method of claim
 17. 32. The structure of claim 31, furthercomprising at least one layer of semiconductor material on the thin filmof the composite structure.
 33. A composite structure comprising atleast one thin film bonded to a support substrate, a bonding layer ofoxide between the support substrate and the thin film, with the thinfilm and the support substrate having a mean thermal expansioncoefficient of 7×10⁻⁶ K⁻¹ or more, wherein the thin film has a thicknessof 5 micrometers or less and the bonding layer of oxide has a thicknessthat is equal to or greater than the thickness of the thin film.
 34. Thestructure of claim 33, further comprising at least one layer ofsemiconductor material on the thin film of the composite structure.